The term "positive type photosensitive lithographic printing plate" generally means a printing plate comprising a hydrophilic support and an ink-receiving photosensitive layer which is formed on the surface of the support and is solubilized by actinic rays such as ultraviolet rays. When the photosensitive layer is imagewise exposed and developed, the non-image portion is removed leaving an image portion. Thus an image is formed. Lithography utilizes difference in the properties of the image portion which is lipophilic and the non-image portion which is hydrophilic.
The photosensitive layer of the positive-type photosensitive lithographic printing plate usually contains an o-quinonediazide compound as a photosensitive component and an alkali-soluble resin which enhances strength and alkali-solubility of the photosensitive layer. Among the o-quinonediazide compounds, 1,2-naphthoquinone-2-diazido-5-sulfonic acid ester compounds are generally preferred because of their high sensitivity and low cost.
Conventionally, developing of such positive photosensitive lithographic printing plates is usually effected with a developer comprising an aqueous alkali solution. The developing ability of a developer is influenced by various parameters and the developing ability falls because of the deterioration by processing a large number of printing plates, degeneration by air oxidation, etc. until the non-image portion cannot be completely dissolved in a rather short period of time. Therefore, photosensitive lithographic printing plates which have a high latitude in developability such that they can be as well developed with a deteriorated developer as developed with a fresh developer. Hereinafter, the limit at which underdeveloping begins is designated by the term "underdevelopability" with respect to photosensitive lithographic plate.
Meanwhile, in the so-called "multi-face exposure process", for instance, in which a photosensitive lithographic printing plate is successively exposed through a plurality of film originals which are placed on the printing plate one by one next to the previously exposed part, the exposed parts should be able to be distinguished from the unexposed parts for film originals to be properly positioned. For this purpose, photosensitive compositions for photosensitive lithographic printing plates usually contain a visiblizing agent pair in order to form visible images upon exposure. This property is hereinafter called "visible-on-exposure characteristic". Said visiblizing agent pair comprises a compound which produces an acid or free radicals upon exposure and a dye which changes color upon interaction with said acid or free radicals. Typical examples of such a compound that produces an acid or free radicals upon exposure are halomethylvinyloxazole compounds disclosed in Japanese Laid-Open Patent Publication No. 55-77742 (1980), for instance, etc. The "visible-on-exposure characteristic generally improves with the increase of the amount of such a compound added. On the other hand, however, deficiencies such as drop of sensitivity of photosensitive lithographic printing plates, fall of solubility with alkali, etc. are caused.
If the amount of the photosensitive component such as 1,2-naphthoquinone-2-diazido-5-sulfonic acid ester compound is increased in order to overcome the above-described deficiencies, the underdevelopability is improved but the sensitivity is lowered. Thus, satisfactory photosensitive compositions are not yet provided.
For better durability and chemical resistance of printing plates, a common novolak resin such as m-cresol novolak resin is used as a binder. In this case, however, printing resistance is not satisfactory when UV inks are used. Therefore, a heating treatment at an elevated temperature called "burning" is usually employed. However, burning tends to cause staining in the non-image portion of printing surface in the course of printing. This is caused by immigration of monomers contained in the resin of the image portion to the non-image portion in the course of burning.
The photosensitive layer using an o-quinonediazide compound, when handled under white fluorescent lamp, suffers from light fogging, and, when subsequently subjected to developing, the photosensitive layer at the image portion is corroded to cause reduction of film thickness, which makes poor the printing performance of the printing plate. Therefore, photosensitive lithographic printing plates having improved resistance to light fogging by white fluorescent lamp (hereinafter called "light tolerance") has been desired.
A photosensitive composition disclosed in Japanese Patent Application Publication No. 52-28401 (1977) comprises an o-naphthoquinonediazide sulfonic acid as a photosensitive material and an acrylic high polymer binder comprising a structural unit represented by the formula ##STR1## wherein R.sup.5 and R.sup.6 are a hydrogen atom or an alkyl group or a carboxylic acid group, respectively; R.sup.7 is a hydrogen atom, a halogen atom or an alkyl group; R.sup.8 is a hydrogen atom, an alkyl group, a phenyl group or an aralkyl group; X is a divalent organic group; n is an integer of 0 or 1; and Y is a substituted or unsubstituted phenylene or naphthylene group. This photosensitive composition contains the above-described high polymer binder instead of a novolak resin and thus is provided with good film strength and printing resistance.
However, it has been revealed that the positive type photosensitive lithographic printing plate comprising a photosensitive layer containing the above-described photosensitive composition disclosed in Japanese Patent Application Publication No. 52-28401 (1977) is still unsatisfactory in UV printing and the developing latitude thereof is low although it exhibits relatively good durability and chemical resistance in ordinary printing. The term "UV printing" means printing conducted using a printing ink containing a UV ray absorber (UV ink). It has been revealed that the reason why said printing plate is not suitable for UV printing is that the chemical resistance of the photosensitive layer to the ink and/or chemicals used in UV printing is poor.
As has been described above, there are known no photosensitive lithographic printing plates which are satisfactory in all of the visible-on-exposure characteristic, development latitude, light tolerance and durability.
Therefore, the purpose of the present invention is to provide a photosensitive composition provided with sufficient performance in all of the above-mentioned visible-on-exposure characteristic, development latitude, light tolerance and durability.